1. Field of the Invention
The invention relates to an apparatus for cutting sheet metal discs from and welding sheet metal discs to sheet metal, which sheet metal may particularly be an endless strip, in a double strip press.
2. Description of the Related Art
Numerous fabricating apparatuses are known which employ sheet metal surfaces as working surfaces. The sheet metal may be in the form of a sheet piece or an endless strip. Generally, the surface of the workpiece has excellent surface uniformity. In the manufacture of, e.g., chipboard, particle board, plastic panels, plastic sheet, and plastic film, etc., the press plates are subjected to high thermal and compressive stresses. Bending of the sheet metal may result even in normal operation. Damage in the border region of the sheet metal may be readily repaired by excising the affected area of the sheet metal and inserting a new piece of metal. Damage in the interior of the sheet metal, which may be a sheet or an endless strip, is repaired similarly, using a discoidal or elliptical or other smooth-contoured piece (hereinafter, "sheet metal disc"), which is welded into place. The smooth contour avoids corners or other discontinuities which may introduce points of weakness.
The procedure used in welding-in such sheet metal discs according to the state of the art is as follows: The region to be replaced is measured, to allow the smallest possible substitution. A hole is drilled or burned in the center of the region to be excised. The course of the cut is marked out using a stencil, compass, or the like. Alternatively, a cutting apparatus, particularly a plasma cutter, can be moved manually along the cut, with the aid of guide means in order to eliminate the marking step. After the region to be replaced is excised in the predetermined dimensions, the cut edge is deburred and cleaned, such as by grinding, but taking care not to enlarge the cut. A pre-fabricated sheet metal piece (e.g., disc) is then positioned in the cut. A backing plate, which may have, e.g., copper inserts, may be used to ensure the planar positioning of the disc at the intended locus of the weld seam. The disc is then welded using a manually manipulated tungsten/inert gas welding apparatus, possibly followed by soft annealing of the weld region. The weld seam is then cold-hardened (e.g., by cold hammering) and is mechanically machined to the thickness of the sheet.
A disadvantage of such an arrangement is that even a small shift in the guiding of the cutting apparatus (e.g., plasma cutter) while excising the damaged region from the sheet metal can result in a mismatch to the replacement piece. Another disadvantage is that irregularities in manual movement of the cutting apparatus can lead to irregularities in the cut region which may weaken the resulting weld. Moreover, the weld quality depends strongly on the manipulative skill etc. of the person performing the weld. Hence, differences in timing of the weld, in particular, may also result in weld seam irregularities, which irregularities can eventually affect wide areas of the sheet metal.
In Ger. Pat. 871,994, a portable cutting apparatus for cutting steel rod and sheet is described. The apparatus employs an electromagnet whereby the apparatus can be held against the workpiece. A hinged straightedge is provided, for accurate positioning of the apparatus. An advantage asserted for this manual burn-cutting apparatus is that no backing is needed. Mechanical drive means may be employed for continuous movement of the burn-cutter.
U.S. Pat. No. 3,417,979 describes an apparatus for cutting and welding which has three adjustable tubular support pieces which are held against a magnetic sheet metal piece (i.e., workpiece) by means of permanent magnets or vacuum holding elements. These support pieces also bear:
guide means in proximity to the sheet metal piece; PA1 gear means (e.g., a toothed rim) above said guide means; and PA1 a support plate disposed along said gear means, which support plate is rotatable by an electric motor. PA1 guide means for a cutting apparatus which guide means can be positioned on or against the sheet metal workpiece; PA1 guide means for a welding apparatus which guide means can be positioned on or against the sheet metal workpiece; and PA1 a backing for the weld seam to be produced; PA1 the guide means are held against the sheet metal strip by means of at least one electromagnet, preferably a plurality of electromagnets, and/or at least one vacuum element, preferably a plurality of vacuum elements, which vacuum element(s) is/are connected to at least one vacuum pump (P); and in that the holding means for the machining and/or cutting and/or welding apparatuses are moveable along the guide means by action of a motor.
Said support plate bears a swingable arm which bears a downwardly directed welding torch, which torch is urged against the guide means by a spring. Accordingly, the welding torch is guided by the guide means, during the rotation of the support plate. Neither permanent magnets, nor the vacuum holding elements provided, are suitable to position the apparatus against the sheet metal workpiece with sufficient accuracy to produce highly accurate guiding of the cutting device or to provide the desired accuracy in the welding of the replacement piece. Whereas, it is stated in said U.S. patent document that the guide means may be used to effect welding onto a sheet metal workpiece, clearly the welding referred to is not welding-in of a replacement sheet metal disc but rather welding for purposes of resurfacing, augmenting thickness, reinforcing, or the like. Such welding does not require the same order of accuracy that is required for welding-in of a pre-fabricated replacement sheet in a recess cut into the sheet metal workpiece by the same apparatus.